1. Field of the Invention
The present invention relates to novel while-drilling Vertical Seismic Profiling (VSP) methods and equipment. One aspect of the invention pertains to measuring and correcting the drift of a downhole clock associated with the VSP receiver in the borehole. Another aspect relates to communication from the surface to while-drilling logging tools in a borehole and to the control of these tools by commands and information sent from the surface by seismic signals and/or by special utilization of surface controllable drilling processes.
2. Description of Related Art
Those in the petroleum industry are increasingly concerned with ‘logging-while-drilling’ (LWD) or ‘measuring-while-drilling’ (MWD) methods that allow early access to information about the geologic and fluid conditions surrounding the borehole as the drilling progresses and during extraction of the drill string from the hole. The LWD and MWD methods are collectively referred to as ‘while-drilling’ methods in this specification. Knowing such information while the drilling process is actively progressing allows decisions to be made that may have a very large impact in terms of attainment of project objectives including petroleum productivity and constraint of project cost. Vertical seismic profiling (VSP) methods have, in the past, been only practicable after the drill string was extracted from the borehole. More recent technology has, to some degree, obviated this requirement. The introduction of several new methods has made it possible to acquire VSP surveys as drilling progresses. However, these new methods have suffered from limitations. One important limitation is the lack of sufficiently accurate time-keeping in the downhole tool: as the downhole clock drifts relative to the master clock at the surface, error accumulates in the seismic travel time measurements that can be very detrimental to the VSP processing and resultant geologic information. Typically, prior art methods have been limited to time-keeping within about 2 milliseconds of true time. VSP surveys could provide significantly better results if time-keeping could be within ½ milliseconds of true time.
The rotation of the drill bit creates seismic energy that can be utilized as a means of illuminating the subsurface geology. If recorded at the surface of the earth, drill bit energy may provide VSP survey information. However this technique does not always work effectively. For example, when drilling soft formations insufficient seismic energy for imaging may be transferred to the in situ formation. Other methods that utilize a surface seismic source combined with seismic receivers in a tool located near to the drill bit in the drill string have been devised and are used. Such methods are designated as ‘while-drilling VSP methods’ in this document. These “while-drilling VSP methods” in the general case and as defined in this specification, may include methods that are applied during pauses in actual drilling and also during pauses in the process of extraction of the drill string or re-insertion of the drill string into the borehole, as is required for replacing drill bits or other reasons.
The seismic receiver package (called VSP receiver hereafter) incorporates seismic and other sensors combined with processing means and is capable of acquiring the VSP seismic data during periods when drilling motion, drilling fluid flow and attendant seismic noise have temporarily abated. The VSP receiver is normally battery powered and is not connected to the surface by wire or fiber conductors that might provide communication to or from the surface.
Complex control signals could readily be conducted to the seismic receiver from the surface by electrical wire or fiber optic link if either were available. However, when a drilling operation is underway it is inconvenient or impractical to provide these physical linkages from the surface to devices deep in the borehole via the drill string.
Other methods have also been sought to transmit control signals to such downhole devices. Electromagnetic communication through the earth between the surface and locations in a borehole has been utilized by the mining and petroleum industries. However, this method is subject to limitations imposed by highly-resistive rock formations and by deep boreholes. Electromagnetic wave signal strength is weakened as formation resistivity in the intervening earth increases. Electromagnetic noise may also prevent successful communication. Hardware in the wellbore such as surface casing and the drill string may interfere with signal reception. Deep boreholes imply high temperature and high pressure conditions, as well as requiring longer signal transmission distances and are not amenable to the application of existing electromagnetic communication systems.
Because of the paucity of opportunity to communicate to the downhole seismic receiver package in prior art while-drilling VSP systems, the seismic receivers have been designed and programmed to operate autonomously, without control by a surface operator for extended periods while downhole. A capability for selective transmission of limited amounts of data (such as observed seismic travel time) from the seismic receiver to the surface via the borehole can be pre-programmed, the corresponding software loaded into a tool at the surface and implemented using an uphole signaling means. One such uphole signaling means is called mud pulse telemetry and utilizes pressure pulses in the circulating borehole fluid (drilling mud) generated by a signal generator device near the drill bit called a mud siren. In conjunction with mud pulse telemetry or other uphole signaling means, a data-on-demand process would be invaluable in that vitally needed data could be requested at any time, but this capability would require a means of sending a command to the mud pulse signal generator tool via the seismic receiver package or dynamically controlling it in some other way.
It would be advantageous to be able to have at least a limited communication with the VSP receiver from the surface while conducting a simultaneous drilling and VSP data acquisition project so that the VSP receiver's operation could be altered in light of new information, such as might be gleaned from the seismic travel times transmitted using mud pulse telemetry. Other LWD and MWD systems also could benefit from such a capability to alter their operations upon demand as drilling progresses. Preferably, the entire drilling and concurrent logging process could be made an adaptive instead of simply a pre-planned operation that is unable to respond to unanticipated drilling conditions and changes in the geologic and fluid models based upon new knowledge gained while drilling. The reward would be in terms of increased probability of drilling success and significantly greater economic return.
As mentioned in the opening paragraph of this section, a synchronization problem has limited the effectiveness and value of VSP surveys conducted with the VSP receiver incommunicado with the surface (except for limited uphole communication via mud pulse telemetry). In seismic imaging and seismic velocity field determination, it is desirable to know seismic travel times to at least the nearest millisecond and preferably to the nearest ½ millisecond. This level of travel time precision necessitates provision of a precision clock as a component of the downhole seismic receiver package. A pre-mission synchronization of the downhole clock with the master clock at the surface ensures that both clocks commence the VSP data acquisition mission in exact agreement as to current time. A post mission re-synchronization allows measurement of total clock drift during the downhole episode and estimates of drift at intermediate times can be interpolated; however this is not sufficiently accurate as the drift rate may not have held constant throughout the downhole mission. The surface clock may be an extremely precise clock because there is no effective limitation on power, cost or physical packaging of the clock, and furthermore it may be periodically updated with other more precise time references such as GPS time. However the downhole clock has limitations imposed because of the environment in which it must operate. Extremes of pressure and temperature in which it must continue to operate with high precision, physical constraints of the deep borehole environment, along with a potentially limited power budget for lengthy downhole missions, have mandated that a clock with less precision than desired must be chosen. While a precision of 10 to the minus 8th power would be considered sufficiently precise for most applications it is not sufficiently precise for the VSP application because it would mean that an error of 1 millisecond could build up in 28 hours. For VSP, the downhole clock needs to be within ½ millisecond of the master clock at all times during the survey and the clock may need to operate downhole for a period of several days or more.
Several prior art methods of synchronizing a downhole VSP clock to a surface clock have been disclosed to utilize sonic signals traveling along a borehole. VSP tools as described in U.S. Pat. No. 5,555,220, or in EP 01464991A(A1), or in WO 00/13043 or in U.S. Pat. No. 6,308,137 have an unfulfilled need for highly accurate synchronization (½ millisecond) of the downhole clock (associated with the downhole seismic receiver) to the surface clock (associated with the seismic source and surface seismic receiver).
U.S. Pat. No. 6,424,595 describes a synchronization method having a “pinger” at the borehole wellhead transmitting signal pulses along a drilling mud column to a downhole “pinger receiver”. Although the '595 procedure accomplishes synchronization, it suffers from precision problems (2 msec) and requires additional equipment (the pinger and pinger-receiver at the surface and downhole). The pinger may not provide sufficient signal strength to allow detection of the reflected pulse and may risk damage to the pipe near the well head that is pinged. Results are not available until the tool is retrieved whereas it is useful and desirable to know the one-way seismic travel time as drilling progresses.
The disclosure of WO 00/13043 describes a method of clock synchronization that includes the transmission of acoustic pulses down a pipe within a wellbore at predetermined times, can also send the current position in the hole to the downhole receiver with acoustic signals, and is able to perform synchronization at the receiver using this approach. Limitations of this solution are accuracy of the acoustic travel time assumption, ability to receive the acoustic signals at significant depth and requirement for the acoustic system (additional equipment and operational considerations).
U.S. Pat. No. 6,308,137 describes a method of seismic signal communication with a downhole well tool. Although the U.S. Pat. No. 6,308,137 disclosure relies upon a high precision downhole clock synchronized to a surface clock, the description includes no accommodation for drift from synchronization.
U.S. Pat. No. 6,002,640 discloses a method of synchronizing a first clock that is associated with a surface positioned seismic receiver to a second clock that is associated with a surface positioned seismic source with the notation that either the receiver or the source may be downhole or in a mine. U.S. Pat. No. 6,584,406 claims an identical method of synchronization but as applied to the case of a downhole seismic receiver associated with a controllable tool. Neither of these two patents specifically describes application of the synchronization method to VSP data acquisition. However, the provisional patent application associated with U.S. Pat. No. 6,002,640 states that the method of the invention can be applied to VSP.
The VSP receiver could also be controlled from the surface, using the methods of these same two patents, to improve its operational performance and capabilities. The same seismic signals could be used to simultaneously control as well as to synchronize the tool. Moreover, these same seismic signals constitute the data that serve the objective of the VSP survey by providing the sought information relevant to the geologic conditions around the borehole. No other method has heretofore been described or patented that can with the very same seismic shots perform these three functions: (1) synchronization of receiver with the seismic source, (2) control of the processes in the seismic receiver and connected tools, and (3) provision of seismic data for the VSP travel time and imaging calculations.
Other MWD tools in proximity to the seismic receiver can also be advantageously controlled by the seismic signals, using the same methods as for control of the VSP receiver. The shots used for control of auxiliary tools may also have the multiple uses described above, i.e. they can also be used at the same time for synchronization of the downhole clock and provision of seismic data for the VSP purposes.
Thus there is a need in the petroleum extraction industry for a method that could overcome the deficiencies of currently available while-drilling vertical seismic profiling systems. This method could provide re-synchronization of the downhole clock while the drill string is in the hole, while simultaneously exercising a wide range of control commands and parameter settings for the downhole VSP receiver and also associated while-drilling logging tools, and would be efficient as well as very reliable. This ideal while-drilling VSP method could also provide information such as seismic travel times, clock drift and an indication of whether a command was received downhole by uphole signaling utilizing mud pulse telemetry or other means.
Certain terms are used in this specification that conform to industry vernacular but require definitions to ensure unambiguous communication. These terms are defined as follows:
SHOT: means a “seismic shot”; used interchangeably with “seismic shot”.
SEISMIC SHOT: defined as (1) the deliberate act of creating seismic energy by a controlled seismic source at a source location in or on the earth; and (2) also is used to refer to the manifestations of that seismic energy as may be received and recorded at various locations away from the site of origin. For example, a “shot” may mean the received and digitized wave energy of the seismic shot as in “the shot was processed by cross-correlating with a prior shot.”SHOT TIME or SHOT INITIATION TIME or INITIATION TIME: defined as the time of initiation of the earliest seismic energy of the seismic shot at the point of origin.SEISMIC TRAVEL TIME or TRAVEL TIME: the time period from the shot initiation time to the time of arrival of the first seismic energy at the VSP receiver in the borehole.SEISMIC SOURCE: refers to the mechanism for creation of the seismic energy. There are two classes of seismic sources, (1) those that are impulsive sources, meaning that substantially all of the energy is initiated in a very short time window, e.g. less than 300 milliseconds, and (2) those that are non-impulsive. The impulsive seismic sources are exemplified by explosive sources and by an airgun source. The non-impulsive sources are typified by the vibratory sources (called Vibroseis in the industry) that create seismic energy continuously over a time period that is typically 5 to 50 seconds in duration. In this document a shot can be initiated by either an impulsive source or a non-impulsive source.SHOT POINT: is the term used to denote the position of the seismic source when a seismic shot occurs.FIXED SITE: an area of limited size in which one or more repeatable seismic sources can be located.REPEATABLE SEISMIC SOURCE: A seismic source that
A. can be activated to transmit a seismic wave form into the earth or into the water layer near the surface of the earth, and
B. can be re-activated again and again, after brief interludes of a few seconds duration, to transmit the same or substantially the same waveform, and
C. whereby the location of the seismic source for the initial activation and for each subsequent activation is substantially the same so that
D. the seismic wave profile from all of the nearly identical transmissions will be nearly identical when observed under sufficiently low ambient noise conditions at a point arbitrarily positioned on or in the earth in proximity to the seismic source.
SUBSTANTIALLY REPEATABLE SEISMIC SOURCE: A repeatable seismic source that achieves nearly identical wave profiles under given conditions wherein cross-correlation coefficients exceed 0.7 and standard deviation of cross-correlation peak times is less than 5 milliseconds.